In a semiconductor power device, a device structure and a device material for minimizing on-resistance and maximizing withstand voltage are required. Conventionally, the semiconductor power device is made of silicon as a semiconductor material, and PN junction formed on a surface referred to as junction termination extension (JTE) and a p-type layer ring structure are made on a portion on which field concentration occurs on a device termination, and it is designed to relax a field, thereby realizing high withstand voltage.
Conventionally, in a Schottky diode, for example, by minimizing the on-resistance and continuously forming a p-layer as the JTE (so-called RESURF layer) from a Schottky electrode portion to the outside, the p-type layer is depleted at the time of inverse bias and the field of the Schottky electrode end is relaxed, thereby obtaining high withstand voltage. The withstand voltage mainly depends on a value obtained by integrating concentration of the p-type layer in a depth direction, that is to say, a dose amount of ion for forming the p-type layer. In order to obtain ideal withstand voltage, it is required that the dose amount is a value near εEc/q when breakdown field strength is set to Ec, a dielectric constant is set to ε, and an elementary electric charge is set to q.
Recently, the power device formed of silicon carbide (SiC) of which performance is extremely beyond that of the power device formed of the silicon has been developed. Since the silicon carbide is a wideband gap semiconductor and of which breakdown field strength is approximately 10 times as large as that of the silicon, this may improve trade-off between the withstand voltage and the on-resistance of the semiconductor power device. In the high withstand voltage semiconductor device formed of the silicon carbide also, as in the silicon, the JTE is formed on the surface to realize the high withstand voltage.
However, since the silicon carbide has anisotropy in the breakdown field strength, the field on an end of the JTE obliquely shifts from a C-axis direction in which the breakdown field strength is the largest, so that there is a problem that the withstand voltage drastically decreases. It is reported that, when setting the breakdown field strength on the C axis (<0001> direction) and an A axis (<11-20> direction orthogonal thereto in which a sign “-” is a “-” (bar) put on a numeral in a field of crystallography) to Ec1 and Ec2, respectively and setting donor concentration in the SiC substrate to Nd, Ec1 and Ec2 are represented by following equations (refer to the Non-patent Document 3). Meanwhile, strictly, the value is an actual measured value of the breakdown field strength in a direction perpendicular to the substrate and in a direction parallel to the substrate when an off angle with respect to the C axis is 8 degrees.Ec1=2.70×106 (Nd/1016)0.1 [V/cm]  (1)Ec2=2.19×106 (Nd/1016)0.1 [V/cm]  (2)
It is understood that the withstand voltage decreases from the ideal voltage on the C axis by 10% or larger in an A axis direction by the anisotropy in the withstand voltage.
In order to prevent the withstand voltage deterioration by the anisotropy in the withstand voltage, the JTE in which the p-layer with further lower concentration is provided outside the p-layer with the low concentration is suggested.